Strictisidine Synthase

 The enzyme strictosidine synthase (STR1 ) (EC 4.3.3.2) from an Indian medicinal plant Rauvolfia serpentina is of primary importance for the biosynthetic pathway of the indole alkaloid ajmaline. STR1 initiates all biosynthetic pathways leading to the entire monoterpenoid indole alkaloid family representing an enormous structural variety of ~2000 compounds in higher plants. The enzyme is involved in the biosynthesis of all these alkaloids by catalyzing the condensation of the two initial building blocks, tryptamine and the monoterpenoid secologanin, leading to the glucoalkaloid strictosidine. The reaction type catalyzed by STR1 is so far an exceptional example in the biosynthesis of natural products. It was hitherto known only from synthetic chemistry (Pictet-Spengler–type reaction), where it is applied in alkaloid synthesis, especially of tetrahydroisoquinolines by condensation of an amine and an aldehyde under acidic conditions.

The overall structure of the enzyme contains a six-bladed four-stranded ß-propeller fold. All six blades are radially arranged around a pseudo six-fold symmetry axis. Each blade contains a twisted four-stranded antiparallel ß-sheet. The substrate binding pocket of STR1 is located near the pseudo six-fold symmetry axis